The invention relates to a shoplifting detection system suitable in particular for the use of high-frequency interrogating signals, in which an electronic label can effect an electromagnatic coupling between two antenna coils, one antenna coil being a transmitting antenna coil fed with an AC interrogating signal from a transmitter circuit, and the other antenna coil being a receiving antenna coil supplying a received signal to a receiver circuit.
Such shoplifting detection systems are known in two types, which can be distinguished on the basis of operation principles, viz. the absorption principle and the transmission principle. In the system that operates according to the absorption principle one and the same antenna is connected to both a transmitter circuit, which generates a high-frequency signal, and a receiver circuit adapted to detect a change in the energy contents of the interrogating signal generated by the magnetic field.
The system operating according to the transmission principle comprises on the one hand at least one transmitting antenna coil, which is connected to a transmitter circuit and which generates an interrogating signal in a detection zone, and, on the other hand, further comprises at least one receiving antenna coil, which is connected to a receiver circuit for detecting a disturbance of the interrogation field.
In both types the electronic label comprises a resonance circuit, which will become resonant at the frequency of the interrogation field. Often the frequency of the interrogation field is periodically varied about the resonance frequency of the label. The presence of an electronic label in the interrogation field then leads to periodic pulse-shaped signals in the receiver circuit.
The invention relates to systems which are based on the transmission principle. A problem in such systems is that the interrogation field itself also generates a signal in the receiving antenna coil which is relatively strong relative to a signal caused by an electronic label. As a result, the sensitivity of such a system is relatively low.
This problem may to some extent be overcome by using antennas of particular shapes as described in, for instance, U.S. Pat. No. 4,243,980 (Lichtblau), or by arranging the transmitting antenna coil transversely to the receiving antenna coil. In practice, however, at the receiving end there is often still a signal component which is directly caused by the interrogation field and whose magnitude, moreover, is strongly dependent on the physical orientation of the antenna coils. As a result, the magnitude of this signal component is not constant, either in time or from one installation to another.
Furthermore, at high frequencies a capacitive coupling is produced, which is frequency-dependent and cannot be eliminated completely or in part in the manner described above